1. Field of the Invention
The present invention relates to a field-effect transistor, a display element, an image display device, and a system. Specifically, the present invention relates to a field-effect transistor containing an active layer composed of oxide semiconductor, a display element and an image display device each containing the field-effect transistor, and a system equipped with the image display device.
2. Description of the Related Art
A field-effect transistor (FET) is a transistor which controls electric current passed between a source electrode and a drain electrode by voltage is applied to a gate electrode to provide a gate for a flow of electrons or holes with applying an electric field to a channel.
The FET has been used as a switching element or an amplifying element, because of properties thereof. Since an FET shows a small gate current and has a flat profile, it can be easily manufactured or integrated compared to a bipolar transistor. For these reasons, the FET is an indispensable element used in many of integrated circuits of current electric devices.
The FET is applied in an active matrix display as a thin film transistor (TFT).
In recent years, liquid crystal displays, organic EL (electroluminescent) displays, electronic paper, and the like have been made into practical use as flat panel displays (FPDs).
These FPDs are driven by a driving circuit containing TFT using amorphous silicon or polycrystalline silicon in an active layer. There are demands for the FPD to be increased in the size, resolution, and driving speed thereof. Along with these demands, TFTs are required to have higher carrier mobility, less characteristic change over time, and less inter-element characteristic variations in a panel.
However, TFTs using amorphous silicon (a-Si) or polycrystalline silicon (particularly, low temperature polysilicon: LTPS) for an active layer have advantages and disadvantages. Therefore, it has been difficult to achieve all of the requirements at the same time.
For example, the a-Si TFT has disadvantages that the mobility thereof is insufficient to drive a large-screen liquid crystal display (LCD) at high speed, and that a large shift of the threshold voltage occurs when being continuously driven. The LTPS-TFT has large mobility, but has problems that variations in threshold voltage is large, as an active layer is crystallized by excimer laser annealing, and a mother glass size of a production line cannot be made large.
Therefore, there is a need for a novel TFT technology, which has both an advantage of a-Si TFT and an advantage of LTPS-TFT. In order to meet this need, development of TFT using an oxide semiconductor, to which carrier mobility superior to that of a-Si can be expected, has been recently actively carried out.
Particularly, InGaZnO4 (a-IGZO), which can be formed into a film at room temperature, and exhibits greater mobility in the amorphous state than that of a-Si, is disclosed (see, K. Nomura and five others, Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors, NATURE, VOL. 432, No. 25, NOVEMBER, 2004, pp. 488-492). Since this disclosure, numerous researches on an amorphous oxide semiconductor having high mobility have been actively conducted.
However, an oxygen concentration of the aforementioned oxide semiconductor needs to be precisely controlled during a film forming process, as carrier electrons are generated by oxygen vacancy. If it is attempted to realize high mobility, the oxide semiconductor tends to be in a depression state, and a process window for realizing normally-off is extremely narrow. Moreover, the oxygen concentration in the film is changed by patterning or passivation process after forming the oxide semiconductor film, and therefore the properties thereof tend to be deteriorated.
In order to solve the aforementioned problems, a countermeasure has been conventionally studied based on two viewpoints. The first viewpoint is to compensate carriers generated by oxygen vacancy with introduction of a p-type dopant (e.g., substituting In3+ with Zn2+) to thereby maintain the carrier concentration low (see Japanese Patent Application Laid-Open (JP-A) Nos. 2002-76356 and 2006-165529). In association with this method, it is also attempted to add a small amount of counter cations to stabilize the p-type dopant (for example, substituting In3+ with Zn2+, and adding a trace amount of Sn4+ ([Zn2+]>[Sn4+])) (see International Publication No. WO2008-096768). The other is a method, in which a certain amount of a metal element (e.g., Al, Zr, and H) having high affinity to oxygen is introduced to prevent generation of carriers (see, J. S. Park, five others, Novel ZrInZnO Thin-film Transistor with Excellent Stability, Advanced Materials, VOL. 21, No. 3, 2009, pp. 329-333).
However, all of the methods had a problem, such as insufficient stability, and low mobility.